As a method replacing a conventional radiography, a radiation image storing and reproducing method utilizing a stimulable phosphor was proposed, and is practically employed. The radiation image storing and reproducing method employs a radiation image storage panel (i.e., stimulable phosphor sheet) comprising a stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (i.e., stimulating light) to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The panel thus treated is subjected to a step for erasing a radiation energy remaining therein, and then stored for the next image storing and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In the method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screen.
The radiation image storage panel has a basic structure comprising a support and a stimulable phosphor layer provided thereon. If the phosphor layer is self-supporting, the support may be omitted. On the free surface (surface not facing the support) of the phosphor layer, a transparent protective film is generally placed to keep the phosphor layer from chemical deterioration or physical damage.
The phosphor layer generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The phosphor layer containing no binder can be formed by deposition process or firing process. Further, the layer comprising agglomerated phosphor soaked with a polymer is also known. In any types of phosphor layers, the stimulable phosphor releases a stimulated emission when excited with a stimulating light after having been exposed to a radiation such as X-rays. Accordingly, the radiation in the form of an image having passed through an object or radiated from an object is absorbed by the phosphor layer of the storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the storage panel in the form of a latent radiation energy-stored image. The latent radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating light. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
Even in the radiation image storing and reproducing method described above, it is naturally desired that a radiation image is reproduced with high sensitivity and with good quality (such as high sharpness and good graininess). Therefore, it is preferred to employ a stimulable phosphor which can efficiently absorb a radiation energy which is applied to the phosphor and releases a high stimulated emission as quickly as possible when a stimulating light is applied.
Until now, a certain number of stimulable phosphors are proposed and some of the proposed stimulable phosphors have been practically employed. The conventional radiation image storing and reproducing method utilizes a system in which a stimulable phosphor absorbs a radiation energy transmitted from the object and then relieves a stimulated emission when a stimulating light is applied.
It is not easy to produce or discover a phosphor which can efficiently absorb a radiation energy which is applied to the phosphor and then releases a desired high stimulated emission as quickly as possible when a stimulating light is applied.
For instance, a rare earth metal activated alkaline earth metal fluorohalide phosphor is well known as a stimulable phosphor which releases a stimulated emission efficiently. However, the fluorohalide phosphor is not satisfactory in the property absorbing a radiation energy when a radiation is applied to the phosphor. Moreover, it does not release the stimulated emission within a desirably short period after it is irradiated with stimulating light. This indicates that the response time of the fluorohalide phosphor is not satisfactorily short.
The problem in the unsatisfactory response time can be solved by the use of a sensor system comprising a multiple number of sensor members such as a line sensor. However, the problem in the unsatisfactory absorption of the radiation energy has not been solved.
Japanese Patent Provisional Publication No. 55-12142 discloses stimulable ZnS phosphors, and Japanese Patent Provisional Publication No. 2-692 discloses an alkali earth metal sulfide phosphors. These stimulable phosphors are still unsatisfactory, as long as the efficiency in absorbing an applied radiation energy is concerned.
Japanese Patent Publication No. 6-31904 describes a radiation image storage panel comprising a stimulable phosphor which absorbs a radiation energy of X-rays and further absorbs a ultraviolet light in the wavelength region of 250 nm to 400 nm and a phosphor which absorbs a radiation having a wavelength of shorter than 250 nm and emits a ultraviolet light in the wavelength region of 250 to 400 nm. Both phosphors are incorporated into one layer or incorporated separately into two phosphor layers. The ultraviolet light-absorbing phosphor serves as an aid to supply the stimulable phosphor with a radiation energy by converting the radiation energy into a ultraviolet light which can be absorbed by the stimulable phosphor.